A role for NO in the control of cardiac metabolism is receiving support from an increasing number of studies. We recently found that the acute blockade of NO synthase (NOS) in normal dogs and the fall in NO production during pacing induced heart failure are both associated with a switch from free fatty acids (FFA) to carbohydrate utilization by the heart. The mechanisms underlying these phenomena are unknown. This research proposal will determine the role of NO in the control of cardiac substrate utilization and whether this role is lost during heart failure. The first specific aim is to determine whether NO controls cardiac FFA and carbohydrate metabolism. The rate of FFA and carbohydrate oxidation in the heart, before and after NOS blockade, will be measured in conscious dogs by infusing isotope-labeled FFA, glucose and lactate. Labeled substrate accumulation and the activity of key enzymes for carbohydrate and FFA oxidation will be measured in cardiac biopsies freeze-clamped at the end of the in vivo experiment. The rapid freezing of the tissue will preserve the activation state of enzymes from in vivo to in vitro. The second specific aim is to determine whether the myocardial metabolic and biochemical alterations occurring during heart failure are similar to those found after NOS blockade in normal hearts and if they can be reversed by a NO-releasing agent. The same methods in vivo and in vitro will be employed. Heart failure will be induced in dogs by chronic pacing. The third specific aim is to determine whether acute alterations of arterial substrate concentration can affect cardiac oxygen consumption during heart failure and if this effect can be reversed by NO-releasing agents. Our preliminary data indicate that FFA consumption can cause increased in cardiac O2 consumption unrelated to hemodynamic changes, but only when NO is absent. These studies will elucidate the role of NO in the pathophysiology of heart failure. This will also provide a new pharmacological mechanism of NO-donors in the treatment of heart failure. based on direct control by these agents on cardiac metabolism.